Solar cells capable of converting sunlight into electric power have been receiving attention as energy sources to replace fossil fuels. Nowadays, solar cells including crystalline silicon substrates and thin-film silicon solar cells are practically used. However, the former solar cells have the problem of high production costs of silicon substrates. The latter thin-film solar cells have the problem of high production costs due to the need to use various types of gases for use in the production of semiconductors and complicated devices. Thus, in any type of solar cell, continuing efforts have been made to reduce the cost per power output by improving photoelectric conversion efficiency. However, the foregoing problems have not yet been solved.
For example, Japanese Patent No. 2664194 (PTL 1) reports, as a new type of solar cell, a photoelectric conversion element on the basis of photoinduced electron transfer in a metal complex. The structure of the photoelectric conversion element is as follows: A photoelectric conversion layer on which a photosensitizing dye adsorbs to have an absorption spectrum in the visible light region and an electrolytic solution are held between two glass substrates. A first electrode and a second electrode are arranged on respective surfaces of the two glass substrates.
The irradiation with light from the first electrode side generates electrons in the photoelectric conversion layer. The generated electrons are transferred from the first electrode to the opposite second electrode through an external electric circuit. The transferred electrons are transported by ions in an electrolyte and return to the photoelectric conversion layer. Electric energy can be taken from the successive transfer of electrons.
The photoelectric conversion element described in PTL 1 has a structure in which a gap between the two glass substrates is filled with the electrolytic solution. Thus, prototype solar cells with small areas can be produced. However, it is difficult to produce a large-area solar cell, for example, a 1 m×1 m square solar cell. That is, in the case of increasing the area of a solar cell, a generation current increases with increasing area. However, the in-plane resistance of the first electrode is increased to increase internal series resistance as a solar cell. This disadvantageously leads to a decrease in fill factor (FF) in current-voltage characteristics during photoelectric conversion.
As attempts to overcome the foregoing problem, for example, Japanese Unexamined Patent Publication (Translation of PCT Application) No. 11-514787 (PTL 2), Japanese Unexamined Patent Application Publication No. 2001-357897 (PTL 3), and Japanese Unexamined Patent Application Publication No. 2002-367686 (PTL 4) report photoelectric conversion element modules each including a plurality of photoelectric conversion elements connected in series. In each of the photoelectric conversion element modules, the increase in internal series resistance is inhibited by electrically connecting an electrode (conductive layer) of the photoelectric conversion element to an electrode (counter conductive layer) of an adjacent photoelectric conversion element.
FIG. 3 is a schematic cross-sectional view of the structure of a conventional photoelectric conversion element. In a conventional photoelectric conversion element 40, a transparent conductive layer 42 is arranged on a transparent substrate 41 as illustrated in FIG. 3. A laminate is arranged on the transparent conductive layer 42, the laminate including a porous semiconductor 43 on which a dye adsorbs, a reflective layer 45, a porous insulating layer 44, a catalyst layer 46, and a counter conductive layer 47 stacked in that order. The transparent substrate 41 and the supporting substrate 48 are fixed with a sealing member 49 in such a manner that a supporting substrate 48 is arranged above the counter conductive layer 47. The laminate is sealed with the transparent substrate 41, the supporting substrate 48, and the sealing member 49. A space in the photoelectric conversion element 40 is filled with a carrier-transport material 51.
In the conventional photoelectric conversion element illustrated in FIG. 3, a material constituting the reflective layer 45 is different from that of the porous insulating layer 44. Thus, delamination is liable to occur between the reflective layer 45 and the porous insulating layer 44. Furthermore, the reflective layer 45 is formed of fine grains having a relatively large size of 100 nm or more and thus has insufficient layer strength. Thus, the delamination is liable to occur between the reflective layer 45 and the porous insulating layer 44.
In Japanese Unexamined Patent Application Publication No. 2010-262760 (PTL 5), the stacking sequence of the porous semiconductor, the reflective layer, and the porous insulating layer is changed in order to prevent the delamination between the reflective layer and the porous insulating layer. The change in stacking sequence inhibits delamination that is liable to occur between layers, thereby producing the photoelectric conversion element in high yield.